The present invention relates to a subsystem replacement technique and, more particularly, to a subsystem replacement operation of a central processing unit in an information processing system which is used on the precondition of stopless operation, or the like.
Data migration denotes a migration of data in an old disk subsystem functioning as an external storage in an information processing system to a new disk subsystem.
Generally, as a method of migrating data between disk subsystems, a migration method using intervention of a central processing unit (CPU) is known. In the method, an access from a CPU to a device as a replacement destination is stopped, and the CPU reads data from an old disk subsystem and writes it into a new disk subsystem. According to the method, however, operations of the user to the disk subsystem are stopped for a long time during the data migration.
As techniques which enable the CPU to make an access also during data migration, there are an HODM (Hitachi Online Data Migration) function of Hitachi Limited, an extended remote copy function (hereinbelow, described as xe2x80x9cXRCxe2x80x9d) and a peer-to-peer remote copy function (hereinbelow, xe2x80x9cPPRCxe2x80x9d) (xe2x80x9cIBM 3990 Model 6 Enhancementsxe2x80x9d) of IBM Corporation, and symmetric data migration service (SDMS) (xe2x80x9cSYMMETRIX ICDA family PRODUCT ANNOUNCEMENT SUMMARYxe2x80x9d, Nov. 6th, 1995) of EMC Corporation.
According to the method of the HODM, an access of the CPU to the old disk subsystem is stopped first. After that, the connection is switched from an access path between the CPU and the old disk subsystem to that between the CPU and a new disk subsystem, and further, an access path is newly provided between the old and new disk subsystems. After that, data of the old disk subsystem is read from the new disk subsystem through the new access path, thereby starting the migration and re-starting the access from the CPU. When the CPU accesses the area from which the data has been migrated, both of the old and new disk subsystems execute processes. When the CPU accesses an area from which the data has not been migrated, data read from the old disk subsystem is reflected also in the new disk subsystem and a process is executed. In this manner, the data migration during an access from the CPU can be realized.
According to the method of XRC, the old disk subsystem has a function of assuring data written from the CPU in a disk controller and the CPU has a function of reading the assured data. By writing the data into the new disk subsystem, the data migration during the access from the CPU can be realized.
According to the method of PPRC, the old and new disk subsystems are connected to each other and are provided with a function of communicating with each other. By writing write data of the CPU to the old disk subsystem to the new disk subsystem by using the communication function, the data migration during the access from the CPU can be realized.
According to the method of SDMS, first, an access from the CPU to the old disk subsystem is stopped. Then, the connection is switched from an access path between the CPU and the old disk subsystem to that between the CPU and the new disk subsystem and, further, a new access path is provided between the old and new disk subsystems. After that, the data of the old disk subsystem is read by the new disk subsystem through the new access path, thereby starting migration. After starting the migration, the access from the CPU is re-started. When the CPU accesses a migrated area, the access is processed directly by the new disk subsystem. When the CPU accesses an area to which the migrating operation has not been executed, data of the track is read from the old disk subsystem and an ordinary process is performed by the new disk subsystem. In this manner, the data migration during the access from the CPU can be realized.
In the above-mentioned methods, by enabling a comprehensive access from the CPU to be performed also during the data migration, stop of an access to the data to be stored into the old disk subsystem can be suppressed to a time for switching from the old disk subsystem to the new disk subsystem. In case of control data of a system such as an OS, even if it is a momentary access stop, it stops the operations of the user and an influence by the migration work is severe. Especially, the access stop is not accepted by the user where 24-hour online operations are necessary. The number of such users is increasing and there is a problem such that the data migration cannot be carried out except for the system stop time such as a winter vacation from the end of the year to the beginning of the next year.
A single subsystem can be used by being connected to a plurality of CPUs. In this case, the subsystem discriminates each of the CPUs on the unit basis of the access path or a group of access paths. An equivalent access to the other subsystem has to be discriminated as well.
When the access path from the CPU is switched to the access path to the new subsystem while making the CPU continuously make accesses, the CPU recognizes that the access is continuously made to the same device. After the data migration is finished and the old subsystem is removed, there is a case that an input request of the device information is issued from the CPU for the purpose of confirmation of the device or the like. In the CPU which confirms the device and the access path in accordance with match/mismatch of device information read in the past with device information read this time. When the information of the new subsystem is sent at this time point, since the device information does not match with each other, it is determined that there is a failure in the access path. Consequently, it is feared that the access path is disconnected and the subsystem becomes down.
It is an object of the invention to provide a subsystem replacement technique which can continue an access from a higher level device to a subsystem also during switching operation from an old subsystem to a new subsystem.
It is another object of the invention to provide a subsystem replacement technique which can migrate data in a stopless state of operations without requiring to stop an access from a higher level device to a subsystem in association with data migration.
It is further another object of the invention to provide a subsystem replacement technique which can smoothly replace an old subsystem operating under the control of a plurality of higher level devices with a new subsystem in a stopless state of operations.
It is further another object of the invention to provide a subsystem replacement technique which realizes smooth subsystem replacement while avoiding occurrence of a failure caused by a change in environment such as device information in association with replacement of an old subsystem with a new subsystem.
Generally, a plurality of access paths are provided from a higher level device such as a CPU or a channel to a subsystem under the control of the higher level device, and the higher level device selectively switches the access paths and accesses to the subsystem. For example, even at the re-start time after interruption occurring in a series of commands issued in relation to an input/output processing request, there is a case such that an access path different from the original access path is selected and used. Since commands before the interruption and those after the interruption relate to the series of the processes, it is obviously understood that if the subsystem has not executed a command before the interruption, the subsystem cannot execute a command after the interruption. According to the invention, even if the access path is changed, the subsystem recognizes the change and can execute the series of commands.
According to the invention, when a plurality of access paths from a higher level device to an old subsystem and an access path between the old and new subsystems are provided, the connection is switched from the access paths between the higher level device and the old subsystem to the access paths between the higher level device and the new subsystem on a plurality of occasions. During the connection change, the access paths are connected from the higher level device to both of the old and new subsystems. In this period, when the higher level device accesses either the old or new subsystem, the accessed subsystem makes an equivalent access to the other subsystem through the access path, thereby relaying the access request. By the operation, the other subsystem executes a command before the interruption so as to be able to execute a command after the interruption.
The equivalent access has to be made by both of the old and new subsystems. When the subsystem which mainly processes the access during the connection change is decided, it is sufficient that the other subsystem makes the equivalent access. When the processing request from the higher level device is not interrupted or when the other subsystem does not receive a command after the interruption from the higher level device such as a case where the processing request from the higher level device is sent via a fixed access path, it is unnecessary to make an equivalent access to the other subsystem.
It is also possible to make an access to the other subsystem through a fixed access path so that the other subsystem does not receive one of successive commands from the higher level device, and to make an access different from that of the higher level device. In this manner, the connection can be switched from the old subsystem to the new subsystem by switching the access paths on a plurality of occasions without stopping the access from the higher level device.
For example, in the data migration in the disk subsystems, when the old disk subsystem is allowed to execute processes dominantly during the connection change and the new disk subsystem relays the access request via a third access path of the invention, the connection can be switched without stopping the access from the higher level device. If the connection is switched while performing the data migration, however, the old disk subsystem is accessed directly by the higher level device during the switching operation and there is a case that data is updated only in the old disk subsystem. When the data of a part which has been migrated is updated, the data of the part is failed to be migrated.
According to the invention, therefore, when the connection switch from the old disk subsystem to the new disk subsystem is realized by relaying an access request sent from the higher level device via a second access path to the new disk subsystem to the old subsystem through the third access path, after completion of the connection switch from the first access paths to the second access paths, the migration of data from the old subsystem to the new subsystem is started. Consequently, it is prevented that the data migrated part is updated not through the new disk subsystem, so that it is unnecessary to perform data migration again.
On the other hand, contrarily, when the new disk subsystem is allowed to execute processes dominantly during the connection change, that is, the old subsystem relays the access request from the higher level device received through the first access path to the new subsystem through the third access path during the connection change from the first access paths to the second access paths, the connection switch can be performed without stopping the access from the higher level device. When the higher level device, however, accesses data which has not been migrated to the new disk subsystem before and during the data migration, it cannot be processed.
Consequently, according to the invention, in this case, prior to the switch from the first access path to the old subsystem to the second access path to the new subsystem, data migration (copy) from the old subsystem to the new subsystem is completed in advance. After that, the old disk subsystem is allowed to relay an access request sent from the higher level device through the first access path to the new subsystem through the third access path, thereby realizing the connection switch from the old disk subsystem to the new disk subsystem.
In order to realize the connection switch during the data migration from the old subsystem to the new subsystem, it is sufficient to always update data of the old and new disk subsystems also during the switching operation.
According to the invention, therefore, in each of the old and new disk subsystems, the access requests received from the higher level device through the first or second access path are relayed to the other subsystem through the third access path, thereby enabling the connection switch during the data migration to be performed.
In the invention, in order to discriminate the access from which one of the plurality of higher level devices and notify the other subsystem of the access, the third access paths of the number equal to or larger than the number of higher level devices connected to the old disk subsystem are provided between the old and new disk subsystems. An access made via the third access path between the disk subsystems is associated with the access from each of the higher level device, thereby enabling the data migration when the old disk subsystem is connected to the plurality of higher level devices to be carried out. The number of the third access paths includes not only the number of physical access paths but also the number of logical access paths.
According to the invention, the new disk subsystem preliminarily issues a device information input request to the old disk subsystem and reads and stores the device information sent from the old disk subsystem in response to the request. In response to the device information input request from the higher level device, not the device information of the new disk subsystem but the stored device information of the old disk subsystem is transmitted.